The subject matter disclosed herein relates to fault detection of series diodes, and more particularly, a method and apparatus for fault detection of series diodes in rectifiers.
Rectifiers are used in a variety of applications to convert alternating current (AC) to direct current (DC), including synchronous electric machines, such as AC motors and generators, which typically have rotating field windings that need to be excited with large direct current (DC) currents in order to operate. To avoid the need to pass these high currents through carbon brushes and slip rings, many of these machines have brushless exciters. These brushless exciters include a rotating set of windings on the machine's rotor that pass through magnetic flux from stationary poles, producing AC current on the rotor. The AC current is then rectified to DC current using a set of rotor-mounted solid state diode rectifiers. Typically, three-phase rotor windings require six rectifier sets to achieve full-wave rectification. For added reliability, each rectifier set is often comprised of two redundant diodes, connected in series, where each diode has the capability to block the full operating reverse voltage by itself when reverse biased.
When series diodes in rectifier sets are functioning properly and reverse biased in the reverse polarity portion of the AC cycle, the pair of diodes will block the flow of current through the diode (while allowing only relatively small amounts of reverse leakage current (mA)) and be subjected to the full reverse voltage across the pair of diodes. A common failure (or fault) of a diode is when the diode allows current that is higher than the normal reverse leakage current to flow when reverse biased. This type of fault can result from a reverse breakdown failure where the reverse voltage damages the diode in some cases to the point where the diode operates as a short circuit when reverse biased. The reverse breakdown failure frequently occurs during transient conditions where the peak reverse voltage exceeds the maximum allowable reverse voltage rating of the diode, which causes a fusion or other breakdown of the diode's PN junction. After a reverse breakdown failure, a diode can no longer effectively block current in the reverse polarity portion of the AC cycle, causing the rectifier current to reach relatively high values and low reverse voltages. Other types of faults (e.g., pinhole shorts) can result in the diode allowing current that is higher than the normal reverse leakage current to flow when reverse biased without necessarily forming a short circuit. As used herein, the term short circuit shall be understood to be a fault condition in a reverse biased diode where the current flowing through the diode is higher than the normal reverse leakage current expected in a properly functioning diode. If only one of the pair of series diodes experiences a short circuit, the other diode can be rated to withstand the full reverse voltage on its own (e.g., 300V to 1000V) and continue to perform the required rectifier function. One problem with the use of redundant diodes is that, if there is no indication that one of the pair of redundant series diodes has experienced a short circuit, the system will continue to operate with the vulnerability that the second diode may fail, increasing the risk of significant damage to the machine. It is, therefore, desirable to have a means of detecting a short circuit in a single diode so repairs can be made on a planned basis and a forced shutdown of the machine upon total failure of diode set can be avoided.
When series diodes in rectifier sets are functioning properly and forward biased in the forward polarity portion of the AC cycle, the pair of diodes will allow flow of current through the diode with a relatively consistent forward voltage (e.g., 0.7V at low current and up to 1.5V at rated current) across the diode. Although less common than a short circuit, another possible fault experienced by a diode is that the diode does not allow the flow of current in the forward direction, causing it to function as an open circuit when forward biased. Even if only one of the pair of series diodes experiences an open circuit, this would inhibit proper operation of that rectifier set, placing greater current demand on the remaining five rectifier sets. It is, therefore, desirable to have a means of detecting the open circuit failure of a single diode so repairs can be made as soon as possible.
Since the series diodes for brushless exciters are mounted on rotating components, the detection of diode faults requires a means of evaluating the condition of the rotating series diodes and then communicating that condition off the rotor. Rotor telemetry is a known technique for communicating electrical signals off rotors by transmitting signals modulated with either analog or digital data from rotor mounted electronic transmitter modules to nearby receivers using, e.g., radio frequency (RF) or optical transmission schemes. These rotor telemetry systems generally also include an inductive power feature that involves coils or antennas, one rotating and one stationary. These coils transmit electrical RF energy from a stationary source to the rotating component, and that energy is rectified to power the rotating transmitter. Typically, the same antenna coil structures that are used for induction power also convey the information signal off the rotor.
One approach to using rotor telemetry to detect diode failures for series diodes on brushless exciters is disclosed in U.S. Pat. No. 6,693,778 B1 to Pittman et al. The diode fault detection system measures the forward voltage across each of the pair of series diodes during the forward polarity of the AC cycle to detect a short circuit. The system compares the measured forward voltage of the diodes to a programmable alarm limit and triggers an alarm that indicates a failed diode or failed pair of diodes. The diode fault detection system is configured to detect short circuit faults only and not necessarily open circuit faults. In addition, measurement of the forward voltage to detect a short circuit can be unreliable since the diode can be damaged by peak reverse voltage in a manner where the forward biased characteristics of the diode are not significantly altered despite the fact that the diode can no longer block current when reverse biased. For example, the forward voltage of a diode with a pinhole short may be almost the same as the forward voltage of a properly functioning diode. Furthermore, since half of the diode fault detection systems are connected to the positive excitation terminal and the other half are connected to the negative excitation terminal, where the terminals have voltage potentials generally hundreds of volts apart, the forward voltage measurement requires two separate modules (one for the diodes on the positive excitation terminal and one for the diodes on the negative excitation terminal), which each need to be powered with an isolated power supply derived from a separate induction power coil.
Another approach to using rotor telemetry to detect diode failures for series diodes on brushless exciters is disclosed in U.S. Patent Application Publication No. 2010/0134075 A1 to Hlavac. The diode fault detection system connects a current source as a sensor across each of the pair of diodes to detect reverse voltage across the each of the diodes. If the sensor detects a reverse voltage across the diode, it is assumed that the diode is functioning, while if the sensor does not detect a reverse voltage across the diode, it is assumed that the diode is not functioning properly. The diode fault detection system is configured to detect short circuit faults only and not necessarily open circuit faults. In addition, making a decision on the presence of a reverse breakdown fault based upon the presence of reverse voltage in excess of a fixed threshold can be unreliable. Since the exciter may be operated over a large range of output voltages and diode characteristics within a rectifier set may be mismatched, a fixed threshold that is set too high may result in failure detection at low operating voltages when none exists and a fixed threshold that is set too low may fail to detect a real existing fault at higher operating voltages.
There is a need to provide improved detection of a diode fault (e.g., a short circuit or open circuit) in redundant series diodes in rectifier sets, including rectifier sets used for synchronous electric machines, such as AC motors and generators.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.